1. Field of the Invention
The present invention relates to an image forming apparatus for electrophotographic system-based image formation, such as a printer, copy machine, facsimile, or the like.
2. Description of the Related Art
For registration correction control of an image forming apparatus, a regular reflective optical sensor is generally used, which includes an LED light source (irradiation optical source) and a PD element (light receiving element). The optical sensor is for detecting a toner pattern for registration correction control. The toner pattern is formed on a base such as a sheet or an image carrier, the image carrier being an intermediate transfer belt, a photosensitive drum, or the like. In the detection of a toner pattern by the sensor, amounts of light reflected by the base and by the toner pattern are detected and a difference therebetween is utilized for position detection. Resultant position information is employed, for example, for adjustment of timings of image formation on photosensitive drums of the image forming apparatus for respective colors.
When an intermediate transfer belt or a photosensitive drum is employed as the base, the gloss of a surface of the base is lowered due to long-term use and contamination thereof. When a sheet is employed as the base, the presence of a variety of sheet types (colored paper, glossy paper, or the like) results in a difference in reflectivity between sheet surfaces. In either case, therefore, the amount of light received from the base can vary. To stabilize the received light amount, a technique to correct the amount (intensity) of light irradiated from the optical sensor has been proposed (Japanese Laid-open Patent Publication No. 6-127039).
When applying such technique to the base having an extremely low reflectivity or having a greatly reduced reflectivity due to long-term use, the drive current of an LED light source of the sensor capable of irradiating a large amount of light is increased to an allowable maximum to stabilize the amount of light reflected from the base.
With increase in the amount of light irradiated from the sensor, however, an amount of irregularly reflected light increases in the toner pattern detection, especially, in the detection of a color toner pattern. The irregularly reflected light amount tends to increase as the toner density increases. At the color toner pattern density equal to or higher than a predetermined density, the irregularly reflected light from the toner pattern becomes strong enough to be received by the sensor. In that case, the amount of light received from a surface of the toner pattern increases toward the amount of light received from the base surface, which reduces the difference between the amounts of light reflected from the base and from the toner pattern. As a result, a failure in the toner pattern detection can be caused, resulting in a fear that a defect image is printed out.
In a regular reflective optical sensor, optical axis misalignment occurs due to a mounting error of the sensor to an apparatus, an assembly error of the sensor, a variation in characteristics of optical parts of the sensor, or other reasons. The optical axis misalignment makes the color toner pattern detection more liable to be affected by the irregularly reflected light, resulting in a variation in sensor characteristic of toner density vs. received light amount, so that the received light amount at a predetermined toner pattern density may be varied. Such variation also occurs due to individual differences between apparatuses or between optical sensors.
Moreover, due to a variation in environment around the apparatus, long-term use of the apparatus, and changeover between image processing modes, the toner pattern density shifts from a target density. With the shift in toner pattern density, the amount of irregularly reflected light from the toner pattern surface also varies, resulting in a variation in the amount of light received by the sensor.
If the amount of light received by the sensor in the toner pattern detection changes to increase, a reduction is caused in the difference between the amounts of light received by the sensor from the base and from the toner pattern, making it difficult to discriminate between the base and the toner pattern. This may result in a failure in toner pattern detection, producing a fear that an image entailing an image position displacement is printed out.